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Research Article

May 2017

Special Issue of International Conference on Emerging Trends in Science & Engineering (ICETSE2017)

Conference Held by IEAE India, at Coorg Institute of Technology, Ponnampet, Karnataka, India

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International Journal of

Emerging Research in Management &Technology

ISSN: 2278-9359 (Volume-6, Issue-5)

Study on the Effect of Heat Treatment on Hardness, Microstructure

and Tribologicalbehaviour of 20MnCr5 Amit Ganapati Shanbhag, Chandrashekhar H., Nithin Holla N. S., P. AkshayPai

Dept. of Mechanical Engineering, Canara Engineering College, Mangaluru,

Karnataka, India

Abstract: Selection of gear material is an important and critical component in automobiles. In this work, it is

basically concentrated on the study on the effect of heat treatment on 20MnCr5 and analyzing its behavior under

different tests. Heat treatment on low carbon steel is to improve its ductility, toughness, strength, hardness and to

relieve internal stress developed. Here among the heat treatment processes case hardening and through hardening is

carried out to suggest the best suitable process considering the time factor also. Further procedural study,

microstructure study, hardness measurement and worn surface morphology is carried out.

Keywords carburizing, hardening, worn surface analysis, microstructure.

I. INTRODUCTION

a) Steel

Steel is an alloy of iron with definite percentage of carbon ranging from 0.15%-1.5%. These plain carbon steels

are changed on the basis of their carbon content as they are major alloying element is carbon. Steels with carbon content

varying from 0.25% to 0.65% are classified as medium carbon steel, while those with carbon content less than 0.25% are

termed as low carbon steel. The carbon content of high carbon steels usually ranges within 0.65% -1.5%. Steel is mainly

an alloy of iron and carbon, where other elements are present in quantities too small to affect the properties. The other

alloying elements allowed in plain-carbon steel are manganese and silicon.

Steel with low carbon content has the same properties as iron, soft but easily formed. As the carbon content

rises, the metal becomes harder and stronger but less ductile and more difficult to weld. Medium carbon steels are used

in mining equipment, and tractors. In addition, machined parts such as bolts, and concrete reinforcing bars are made of

this class of carbon steel. Gears, wire rods, seamless tubing, hot-rolled/cold-finished bars and forging products are some

of the objects constructed from medium carbon steel.

The material modification process modifies the behaviour of the steels in a beneficial manner to maximize

service life i.e. stress relieving or strength properties e.g. cryogenic treatment or some other desirable properties. Heat

treatment is a combination of timed heating and cooling applied to a particular metal or alloy in the solid state in such

ways as to produce certain microstructure and desired mechanical properties (hardness, toughness, yield strength,

ultimate tensile strength, youngs modulus, percentage of elongation and percentage of reduction). Annealing,

normalizing, hardening and tempering are the most important heat treatments often used to modify the microstructure

and mechanical properties of steels.

Heat treatment involves the application of heat, to a material to obtain desired material properties. During the

heat treatment process, the material usually undergoes phase micro structural and cryptographic changes. The purpose of

heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength,

tensile strength and impact resistance.

II. LITERATURE SURVEY

a) Literature review

GUPTA et al. [1] studied the effect of sliding wear characteristics of 0.13wt % carbon steel. Heat treatment was

carried out under various conditions, monitored on a standard pin-on-disk wear testing machine under the normal loads

of 2.5kg, 4.5kg and 5.5kg and at a constant sliding velocity of 1 m/s. Weight loss of the specimen was measured at

various time intervals to obtain wear rate. They found out that the volume loss increases with sliding distance. The

mechanism of wear is primarily oxidative, ferrite coarse pearlite is superior to ferrite fine pearlite at all loads. Based

on the wear coefficients the ferrite coarse pearlite, ferritefine pearlite, ferritetempered martensite and ferrite

martensite structures in 0.13 wt. % carbon steel show the wear resistance in decreasing order.

KADAM et al. [2] performed research study on the sliding wear behaviour of carburized 20MnCr5 alloy steel.

The abrasive wear study is performed on pin-on-disc tribotester. Sliding velocity of 0.8m/s, 1.6m/s, 2.4m/s, sliding

distance of 1200m, 1400m, 1600m and Load of 3kg, 4kg and 5kg were used to evaluate the wear resistance. They found

that minimum wear of treated samples is obtained when load is 4kg sliding distances 1200m and sliding velocity is

2.4m/s.

P. STRATTON. [3] studied the effect of cold treatments on the lubricated wear case hardened components. The

samples were cooled to -269C for 168 hours in liquid helium. The outcome of the study was such that it improved the

parameters that affect component life and decreased the wear. The colder the treatment, the more retained austenite is

transformed and improves the wear resistance.

Shanbhag et al., International Journal of Emerging Research in Management &Technology



AMANDEEP SINGH WADHWA et al. [4] compared the surface hardening techniques for En353 steel grade.

In this paper digester screw made of En353 steel grade is subjected to four surface hardening techniques namely pack

carburizing, gas carburizing, tungsten carbide coating using thermal spray and Stellite 6 coating using TIG welding.

Results and conclusions were then drawn by performing wear and hardness tests.

b) Objective

Having the above data in hand, the project was started with the following objectives.

Evaluation of mechanical properties (Hardness and Sliding Wear) of 20MnCr5.

To access a suitable heat treatment process.

To carry out microstructure study of each sample before and after heat treatment and to analyze the change in microstructure.

Analysis of the worn surface morphology.

c) Methodology

20MnCr5 of (1230) mm was the selected material.

Following heat treatment processes were selected. 1) Case Hardening. 2) Hardening and Tempering.

The following tests were selected for studying the change in properties. 1) Hardness test: Measuring the surface hardness of the sample using C scale of the Rockwell hardness tester

(HRC).

2) Wear test: Measure the wear that takes place in each sample, which will be done using the wear testing machine.

3) Microstructure analysis: Microstructure of the samples before heat treatment (BHT) and after heat treatment (AHT) and analyze the same.

4) Wear surface morphology: Capturing the images of the worn surfaces using an optical microscope in order to assess which wear mechanism dominates among the different hard facing alloys.

III. MATERIALS AND EXPERIMENTATION

a) 20MnCr5

20MnCr5 is known as special structural steel (low-carbon, low-chromium steel), and may be recommended for

the applications where wear resistance, medium strength and good toughness are required. In this sense it is used

mechanical engineering for highly stressed components, for example in automobile industry, for parts like gears,

crankshafts. It has a low carbon content and can be hardened by heat treatment followed by tempering and quenching to

achieve the required HRC. The following data sheet gives an overview of 20MnCr5.

Chemical composition of 20MnCr5:

Table 1: Chemical composition

Element Content (%)

Carbon, C 0.18-0.19

Silicon, Si 0.18-0.19

Manganese, Mn 1.28-1.29

Sulphur, S 0.013-0.015

Chromium, Cr 1.00-1.30

Phosphorous, P 0.016-0.018

b) Specimen preparation

The first and the foremost thing for the experiment is the specimen preparation. The specimen size should be

compatible to the machine specifications. The raw sample procured from the steel trader was Low carbon steel 20MnCr5

having 0.18% of carbon. The dimension of prepared specimen is given below:

For hardness test (Vickers Hardness Test) and for wear test (Sliding Wear Test)

Figure 1: Specimen prepared for hardness and wear test




c) Procedure

Carburizing

Initially the raw material was machined to the required specification (1230mm). This was done to make sure that the component can be tested for wear analysis.

The samples were then loaded to the furnace and heated to autenizing temperature and exposed to carbon rich atmosphere.

It takes 6 hours to attain 930C with a carbon potential of 0.8%. After attaining 930C, the samples were heated in the furnace with 4 hours for activation and 5 hours for diffusion processes to take place with the carbon

potential (CP) 0.8% and 0.4% respectively.

Then the furnace temperature is lowered to 820C and the CP maintained is 0.4%.

Finally the specimens were removed from the furnace and oil quenched for 20min followed by tempering at 180C for 3 hours.

Figure 2: Graph plotted for carburising process time

Figure 3: Case hardened specimen

Hardening

The specification of the specimens being the same as that for case hardening, another set of samples were heated in the furnace up to 860C.

It takes 5 hours to attain the temperature of 860C, then maintained at the same temperature for 2 hours with the CP 3% to achieve hardening heat treatment process.

After completion of hardening heat treatment process, the temperature is lowered and specimens were oil quenched and then followed by tempering at 180C for 3 hours.

A carbon rich atmosphere in the furnace was achieved with the help of methanol (CH3OH).




Figure 4: Graph plotted for hardening process time

Figure 5: Through hardened specimen

Figure 6: Tempering process time




d)Hardness testing

The heat treated specimens hardness was measured by means of Rockwell hardness tester. The procedure adopted was

listed as follows:

First the diamond indenter was inserted in the machine.

A load of 150kgf was used for every test.

The load was made to dwell on the specimen for a period of 5 seconds.

Once these settings were configured, the test was started and the values observed in the dial were noted down.

Figure 7: Rockwell hardness apparatus

e) Microstructure study

The change of microstructure in the material due to heat treatment is the main reason for the improved

mechanical properties. Hence the microstructure examination was carried out to find the structure of BHT and AHT.

This consisted of the following steps:

Rough and fine grinding of the specimen to remove the scales or scratches on the surface.

Polishing the specimen on velvet cloth until a mirror like surface is obtained.

Etching the specimen so that the microstructure would be visible under the microscope. This consists of dipping the sample in 3% nital solution for 2 to 3seconds and then cleaning it with ethyl alcohol for the carburized

specimen. Similar etchants are used for the other specimen.

Observing the microstructure under a metallurgical microscope.

f)Sliding wear test:

Sliding wear test was carried out in order to find out the loss in weight after a specific period of time. Wear is

related to interactions between surfaces and specifically the removal and deformation of material on a surface as a result

of mechanical action of the opposite surface. The need for relative motion between two surfaces and initial mechanical

contact between asperities is an important distinction between mechanical wear compared to other processes with similar

outcomes. Among many failure modes associated with steel components, wear presents a unique challenge. In the

present investigation, an attempt was made to study the wear characteristics under dry sliding condition at different loads

and speed with fixed distance at room temperature, using a standard pin-on-disk wear testing machine.

For this the following steps were adopted:

Process starts by weighing the specimen on a sensitive milligram scale. This was done so that the weight loss due to wear can be determined

The specimen was fixed onto the machine which has a spinning metallic disc, thereby need to make sure that the fixed sample is properly levelled.

A constant and varying load was applied on the specimen at a constant and varying speed. The track diameter was set at 80mm.

Wear testing was done on the specimen for 15min under dry conditions after which it was taken out of the machine.

Finally, measure the weight of the wear tested specimen. Difference between the initial weight and final weight of the specimen gives the wear loss in 15minutes.




Table 2: Specifications of the wear testing machine

Sl. No. Description Details

1 Speed Min 200 rpm, max 2000 rpm

2 Normal load 200N max

3 Frictional force 200N max

4 Wear 2 mm

5 Wear track diameter Min 50mm, max 100mm

6 Sliding speed Min 0.5 m/sec, max 10 m/sec

7 Specification size(pin) 3,4,5,6,8,10 & 12mm

8 Wear disc EN 31, Hardened to 58 to 62 HRC

Figure 8: Pin on disc tester

g)Test parameters and their range:

Wear test was conducted according to the following test parameters for the duration of 15 minutes.

Table 3: Wear test parameters

Distance Load Speed

Sl. No. (m) (Kgf) (rpm)

1 600 2 450

2 600 4 450

3 600 6 450

4 600 8 450

5 600 10 450

6 600 6 150

7 600 6 300

8 600 6 600

9 600 6 750




h) Worn surface morphology

In order to study the morphology and to assess which wear mechanism dominates among the different hard

facing alloys, the micrographs of the worn surfaces of the specimens were examined. The images of the worn surfaces

were captured using an optical microscope with the magnification of 100X, 200X and 500X as to determine how the

wear on each specimen has taken place. The occurrence of different types of wear mechanisms may depend upon the

chemical composition, microstructure and hardness of the material.

IV. RESULTS AND DISCUSSION

Various mechanical tests were conducted on the specimens were studied and analysed. For carburized

specimens, the case depth obtained was 1.6mm and the optimised process among hardening and carburizing heat

treatment is suggested.

i) Effect of heat treatment on hardness

The load applied for the following values is 150kgf and the dwell time is 5 seconds.

Figure 9: Comparison of hardness by heat treatmen

The above graph shows the variation in Rockwell Hardness Number (HRC). Case hardened and hardened

sample has the highest hardness compared to the other two samples. It is clearly notices that the base material (BHT) has

the low hardness. The AHT specimen has high hardness compared to the BHT specimen. Hence case hardening gives a

good surface hardness as compared to the other procedures followed. Further it is a good method for obtaining surface

hardening for low carbon alloys.

j) Microstructure analysis:

In order to look at the microstructure, first fine grind the sample and then polish it. Then etch the sample and

observe its microstructure. The magnification of the images is 100X and 500X. All the three specimens were selected for

the microstructure analysis. The effectiveness of the heat treatment process in refining and homogenizing the structure

for 20MnCr5 steel is shown in following figures.

Figure 10: Marble etched plain specimen (100X)






Figure 13: Etched Hardened specimen, T = 860C, t = 10hours (100X)

Figure 14: Etched Hardened specimen, T =860C, t = 10hours (500X)




Figure15: Etched Case Hardened specimen, T= 930C, t = 20hours (100X)

Figure 16: Etched Case Hardened specimen, T = 930C, t = 20hours(500X)

Figure 17: Etched Case Hardened specimen, T=930C, t = 20hours (500X)

Figure 18: Etched Case Hardened specimen, T = 930C, t = 20hours (500X)

Raw material shows equiaxed grains in figure 10, figure 11 and figure 12.

Hardened specimen shows the formation of martensite along with retained austenite in figure 13 and figure 14.

Carbon percentage is increased after carburizing heat treatment and the formation of carbide network is observed in the figure 17 and figure 18.

Formation of carbide network is more in case hardened specimen which results in brittleness of the material and unsuitable for machining operations. Hence the specimen has to be carburised for lesser cycle time. To attain

this diffusion time has to be reduced and tempering temperature need to be increased.




k) Sliding wear behaviour

Wear test was conducted on non-heat-treated and heat-treated cylindrical specimens of size 30mm length and

12mm diameter. The wear test was carried out against the counter face of a hardened and polished disc made of En31

steel having HRC from 58 to 65 at a relative humidity of 51.5%. Pin on disc machine in DUCOM, Bangalore (INDIA)

was used to carry out the test.

l) Effect of Load

Figure 19: Volume loss vs speed graph keeping speed and distance constant

m) Effect of Speed

Figure 20: Volume loss vs speed graph keeping load and distance constant

Material with high hardness value will have high wear resistance. Greater hardness does not always mean

greater wear resistance or longer life as it depends on the specimen composition and testing parameters. Several alloys

may have the same hardness rating but vary greatly in their ability to withstand wear. The curve for the carburised

specimen in the figure 19 and figure 20 shows minimum wear as compared to hardened and before heat treated

specimens for varying load and speed respectively.




n) Wear surface analysis

Before heat treatment

Figure 21: Min load (2kg) Figure 22: Max load (10kg)

Figure 23: Min rpm (150) Figure 24: Max rpm (750)

Hardened specimen






Carburized specimen



From the Figure 21 and 22 it can be observed that, the micrograph of un-hard faced specimen shows much

deeper and wider grooves along the wear track which resulted in more damaged regions. This specifies the ploughing

type of wear mechanism is prevailing in un-hardfaced of 20MnCr5 steel. In comparison to this, as shown in Figure 25

and 26 significantly shallower, finer and more continuous wear grooves along the wear track indicate that micro-cutting

is the dominating wear mechanism in hardened sample. Figure 29 and 30 shows comparatively less deep and wider

grooves than un-hardfaced one, which also indicates a ploughing type wear mechanism. The un-hardfaced substrate

contains a matrix of ferrite which has higher ductility and lower hardness, causing ploughing to become the dominating

wear mechanism. However, in hardened specimen cutting and ploughing mechanisms, were taking place simultaneously.

In hardened specimen, cutting to be the dominating wear mechanism owing to hard carbides.

For minimum load with constant speed of 450rpm the wear loss is less along the wear direction for all the three specimens.

For the plain specimen with high load applied the counter face is in high contact with the wear disc and wear

rate is very high in this condition.

For maximum load with constant speed of 450rpm the wear loss is more along the wear direction. Formation of

wear debris (oxides and hard carburized particles) and the surface metal tends to fold along the wear direction.

Severe plastic deformation has taken place due to high load applied and also formation of cracks can be

observed on the material surface.

For minimum speed with constant load of 6kg the wear loss is less along the wear direction for all the three

specimens. Here we can clearly see the wear debris.

For maximum speed and constant load of 6kg the wear loss is more along the wear direction. Surface metal is

folded and is worn out as speed is high.

And finally analysis regarding the effectiveness of the heat treatment process, we can see that plain specimen

wears fast than hardened and case hardened specimen. As hardness is less in hardened specimen compared to

case hardened, the material wears out and cannot withstand high load and high speed.

Whereas the case hardened specimen with a hardness value of 62HRC has the best stability against wear and

therefore resistivity of the surface hardness leads to formation of crack has taken place in the specimen.




o) Chemical composition after heat treatment

Figure 33: Variation in the percentage of carbon

From the AHT chemical composition it is observed that carbon percentage has increased by 0.02% for hardened

sample and for case hardened specimen it has increased by 0.34%

V. CONCLUSION

Following conclusions are drawn from the results.

Highest hardness is obtained fr the case hardening treatment.

Case hardening process improves the microstructure and wear resistance of the material, due to reduced free ferrite. Carbon percentage is increased after carburizing heat treatment.

Hardening heat treatment also increases the hardness of the material but is less compared to the case hardened specimen. Hence hardened specimen wears easily when compared with the case hardened specimen.

Microscopic examination revealed the fact that there exists the amount of cementite along with the martensite. The formation of carbide network in case hardened specimen is more. In order to reduce the carbide network,

diffusion time has to be reduced and tempering temperature need to be increased.

REFERENCES

[1] V. K. Gupta, S. Ray, O. P. Pandey, Dry sliding wear characteristics of 0.13 wt.% carbon steel,Materials Science-Poland., vol. 26, no. 3, pp. 618-631, 2008.

[2] S. S. Kadam and P. S. D. Ambekar, Dry sliding wear behavior of carburized 20MnCr5 alloy steel,International Journal of Science and Reasearch., vol. 3, no. 5, pp. 6772, 2014.

[3] P. Stratton, The effect of cold treatments on the lubricated wear of case hardened components, pp. 16, 2010.

[4] A. S. Wadhwa and S. Akhai, Comparison of surface hardening techniques for En353 steel grade,International Journal of Emerging Technology and AdvancedEnginnering., vol. 4, no. 10, pp. 194-203,

2014.

[5] S. Kumar, A. Jain, and P. Singh, Analysis of abrasive wear characterization and its correlation with structure for low and medium carbon steels,International Journal ofEmerging Technology and Advanced Engineering.,

vol. 2, no. 12, pp. 782790, 2012.

[6] A. Singh, G. Singh, and S. Singla, Wear behavior of hardfacings on rotary tiller blades, Procedia Engineering., vol. 97, pp. 14421451, 2014.

[7] X. Steel, J. Brnic, and M. Brcic, Comparison of mechanical properties and resistance to creep of 20MnCr5 and X10CrAlSi25 steel,Procedia Engineering., vol. 100, pp. 8489, 2015.

[8] R. Sola, R. Giovanardi, P. Veronesi, and G. Poli, Improvement of wear and corrosion resistance of ferrous alloys by post-nitrocarburizing treatments,Metallurgical Scienceand Technology., vol. 29, pp. 14

24, 2011.

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Conference Held by IEAE India, at Coorg Institute of ...ermt.net/docs/papers/Special_Issue/2017/ICETSE/101P.pdf · 20MnCr5 is known as special structural steel (low-carbon, low-chromium